This invention relates to thermal ink jet printing and, in particular, to providing redundant printing capability for improving the reliability of a printer.
Printers using full width printheads (i.e., printbars) are known to offer several advantages over conventional printers in which a single printhead travels back and forth across the printing medium. The advantages of full width (or page width) printheads include faster printing speed, improved reliability, and quieter operation.
Nevertheless, full width printheads using thermal ink jet technology suffer from a drawback. A full width printhead may include 7200 or more discrete marking elements (i.e., ink jets), each of which must function properly to ensure that a high quality image is produced. Having such a great number of discrete ink jets increases the probability that any single ink jet will fail. Since no practicable method exists for repairing ink jets, the failure of a single ink jet requires the replacement of the entire full width printhead. In addition to imposing an undesirable expense, a nonfunctioning full width printhead results in a considerable loss of printing time and inconvenience while a new full printhead is obtained and installed.
As known in the prior art, failed ink jets can be detected through the use of a drop sensor that recognizes missing or misdirected drops. Several drop sensing devices use a light beam that projects across the width of the printing medium and between the printhead and the printing medium to a detector. Based upon the timing and degree of oclusion caused by an ink droplet passing through the light beam, the devices can sense the size and directional accuracy of the ink droplets. A laser may also be used to generate the light beam. For example, Japanese Laid-Open Patent Application No. 4-315914, assigned to Fujitsu Ltd., discloses a method of detecting failed ink jets by comparing variations in the flight angle and flight time of each ink droplet to known values. Other examples of drop detecting devices and methods are disclosed in U.S. Pat. No. 5,179,418 and Japanese Patent Application No. 4-276446.
Conventionally, a full width thermal ink jet printhead usually comprises a number of individual ink jet print dies attached to a substrate that serves as a heat sink. The individual print dies, which each contain up to several hundred individual ink jets, can be attached to the substrate according to a number of different configurations. The individual printheads can be disposed in a linear array such that each printhead is in contact with each adjacent printhead. Alternatively, the individual printheads can be spaced apart along both sides of the substrate in a staggered pattern such that each printhead is attached to one side of the substrate and disposed opposite a space on the other side of the substrate. Accordingly, the printheads on either side of the substrate cannot print a continuous line of text, but all of the printheads on both sides of the substrate, taken together, produce a continuous line of text across the width of a recording medium. For other variations of full width printhead configurations, see, e.g., U.S. Pat. No. 5,057,854, issued to Pond et al., the disclosure of which is hereby incorporated by reference.
For color printing applications, several full width printheads are often used in conjunction with one another. Each full width printhead is separately supplied with ink of a different color. In the most common configuration, four full width printheads are used to print the primary colors of black, cyan, magenta, and yellow inks. Each of the four full width printheads is disposed above the surface of the recording medium, perpendicular to its direction of travel and parallel to the other full width printheads.
Even in color printers, however, most printing requires the use of black ink. As a result, a full width printhead for printing black ink is more likely to fail than the other full width printheads. Considering the impracticability of repairing a full width printhead and the importance of ensuring high reliability, providing a redundant configuration in the case of a failed full width printhead would be advantageous.
From a theoretical perspective, the increased reliability of a printer having a redundant configuration can be expressed according to known mathematical relationships. Assuming the phenomenon of ink jet failure to be random, F represents the failure rate for an individual jet. Accordingly, for N printheads, the probability that a particular location on the printhead will have at least one operating jet is: EQU (1-F.sup.N)
If there are n jets on a printhead, the probability P that the ink jets at all locations are functional is: EQU P={(1-F.sup.N)}.sup.n
Assuming that N is 7200 (i.e., that there are 7200 ink jets on the printhead) and P is 0.95 (i.e., only one printhead out of 20 fails), the individual ink jet failure rate F can be expressed as a function of the number of full width printheads:
______________________________________ Individual Failure Rate, F Number of Print Bars, N Over Printhead Life ______________________________________ 1 0.0000071 2 0.00267 3 0.0192 4 0.0517 ______________________________________
As shown above, if four full width printheads are used, more than 5% of the individual jets can fail before the printing reliability in a monochrome mode drops below 95%.